Activated carbon is a nongraphitic, microcrystalline form of carbon which has been processed to produce a carbon with high porosity. The pores formed in the activated carbon may be macropores (e.g., pores having a diameter greater than about 500 angstroms), mesopores (e.g., pores having a diameter between about 20 and 500 angstroms), or micropores (e.g., pores having a diameter less than about 20 angstroms). Activated carbon is characterized by a high specific surface area (e.g., 300 to 2500 m.sup.2 /g) and is known for its high adsorptive capability. Activated carbon enjoys widespread use in the removal of impurities from fluid (i.e., liquid or gas) streams. For example, impurities in foods (e.g., fruit juices, alcoholic beverages) or medicinals (e.g., insulin, vitamins) can be successfully filtered using activated carbon. Likewise, activated carbon is useful in the removal of gaseous species present in low concentrations in air or gas streams (e.g., in gas separation processes, processes for removal of organic vapors, or in cigarette filters). Activated carbon has particular utility in adsorbing and purifying fluid emissions from internal combustion engines.
Conventionally, activated carbon is used in a powdered or granular form. Powdered or granular activated carbon is inconvenient to use in processes where continuous flows of fluids are filtered and/or treated. To solve this problem, attempts have been made to use activated carbon in the form of, or in conjunction with, a solid substrate.
For example, attempts have been made to manufacture monolithic substrates consisting essentially of activated carbon or to extrude carbonaceous material as a substrate and then convert the entire substrate to activated carbon. In such processes, a binder is typically added to the activated carbon powder and the mixture is extruded as a monolithic substrate. See, for example, U.S. Pat. Nos. 5,043,310 to Takeuchi, et al., 4,999,330 to Bose, et al., 4,399,052 to Sugino, and 4,386,947 to Mizuno, et al. Substrates formed by these methods have limited utility. For example, the binder used to facilitate extrusion will block the pores of the activated carbon and, therefore, diminish the substrate's adsorption capability. If the amount of binder is reduced to minimize blocking, the strength of the substrate is unacceptably reduced. Furthermore, most substances useful as extrusion binders begin to deteriorate at temperatures above 150.degree. C., further diminishing their applicability. Lastly, components of the process stream being filtered often react with commonly used extrusion binders, causing the binder to deteriorate during use. For example, water present in a fluid stream will dissolve methylcellulose, a very commonly used extrusion binder.
U.S. Pat. No. 4,518,704 to Okabayashi, et al. describes a method for making an activated carbon substrate using an inorganic binder (e.g., clay, talc, alumina, fusible glass powder). The high percentage of binder particles required to achieve minimal strength in the honeycomb, however, results in low adsorptive capability. Furthermore, the strength of the formed substrate remains low due to the poor bonding of the carbon to the inorganic binders.
Other, likewise unsatisfactory, attempts to form activated carbon substrates feature coating a substrate with, for example, a slurry of activated carbon in a binder. See U.S. Pat. Nos. 4,992,319 to Kurosawa, et al. and 5,104,540 to Day, et al. The requisite binder in the carbon coating results in substrates with poor adsorptive capability due to the binder particles closing off some of the porosity in the activated carbon. Furthermore, the activated carbon is prone to flaking or chipping off the substrate due to the weak bond between the binder, the carbon, and the substrate. Therefore, there continues to be a need for substrates utilizing activated carbon that are strong, temperature resistant, resistant to chipping or flaking, and highly adsorptive.